The invention relates to a process for the preparation of residual fuel with low sulphur content from asphalt.
As a rule, asphalt has too high a viscosity and too high a sulphur content to be used as fuel oil without modification. Both the sulphur content and the viscosity of asphalt can be reduced by mixing the asphalt with a distillate having a low sulphur content or by subjecting the asphalt to a catalytic hydrotreatment. If the difference in price between residual fuel having a low sulphur content and distillate having a low sulphur content were the only criterion in deciding which of the two solutions should be preferred, the second solution would undoubtedly have to be chosen. For, when asphalt is mixed with a distillate having a low sulphur content, a considerable quantity of relatively valuable distillate disappears into the asphalt and is sold at the residual fuel price, whereas the catalytic hydrotreatment of the asphalt yields, in addition to residual fuel having a low sulphur content as the main product, a considerable quantity of distillate having a low sulphur content as a by-product.
Processes for hydrotreatment of asphalt in the presence of a hydrogen donor are described, e.g. in U.S. Pat. No. 3,407,134 and German Pat. No. 1,034,302. Further a process for catalytic hydrogenation of heavy oil with catalyst circulating between two reactors is disclosed in applicants' recently issued U.S. Pat. No. 4,073,721.
However, a drawback of the catalytic hydrotreatment of asphalt is that it has to be carried out at a high hydrogen partial pressure, in order to avoid rapid catalyst deactivation. Generally, it may be said that in the catalytic hydrotreatment of asphalt for the preparation of residual having a low sulphur content, hydrogen partial pressures higher than 100 bar (1450 psi) should be used to reach a catalyst life acceptable for commercial application. The use of such high hydrogen partial pressure requires expensive equipment and involves a high hydrogen consumption. In view of the ample availability of asphalt and the increasing demand for residual fuel having a low sulphur content, there is an urgent need for a process permitting the catalytic hydrotreatment to be carried out at low pressure.
An investigation concerning this subject has been carried out by the Applicants. In this investigation it was found that using a hydrogen donor in combination with reactivation of the catalyst and hydrogenation of the hydrogen donor it is possible to realize a process suitable for use on a commercial scale, in which the hydrotreatment of the asphalt can be carried out at hydrogen partial pressures below 50 bar (725 psi). In this process two reactors are used, the hydrotreatment of the asphalt being carried out in one reactor and the reactivation of the catalyst and the hydrogenation of the hydrogen donor in the other reactor.